The present invention relates generally to an apparatus and a method for nondestructive testing of objects, and more particularly to an apparatus and method for utilizing low frequency radiation in the production of fringe patterns or synthetic holograms which are capable of being reconstructed by holographic techniques.
This disclosure relates generally to nondestructive testing for identifying structural characteristics of an object by scanned holographic techniques using a known source of radiation, such as electromagnetic or acoustical radiation. While electromagnetic radiation and nondestructive eddy current techniques are used in the described illustrative embodiments of the invention, it will be understood by those skilled in this technical area that other forms of radiation can be substituted and are to be encompassed within the disclosure of this invention.
The principles of electromagnetic nondestructive testing are well known. Specifically, eddy currents are generated within an object to be inspected by induction from an adjacent coil by an alternating excitation current. Eddy currents then generate magnetic fields which couple to the coil at the same frequency as that of the excitation current, but which may be of a different phase. The phase and amplitude of the induced voltages depend upon the structural characteristics of the object under test. The phase relationships may be measured by appropriate signal processing circuits.
The flow of eddy currents in a test object is governed by the skin effect phenomenon. The currents decrease exponentially with depth, depending on the shape of the object, its thickness, and its electromagnetic properties. In addition to the decrease of current amplitude as depth below the surface increases, the phase angle of the current increasingly lags the excitation signal.
A detriment common to many eddy current test procedures is the inability to obtain sharp dimensional definition of flows or anomalies. When images are obtained, flaws in the object being tested are typically portrayed in a shapeless image or an image whose shape does not correlate to the shape of the flaw itself. This limitation arises from the long electromagnetic wavelengths required to obtain adequate penetration into the object. If wavelength is reduced by increasing the test frequency, eddy current penetration is likewise reduced because of the skin effect phenomonen. Furthermore, geometric limitations in the size of the available scanning aperture prevents imaging of flaws by holographic imaging processes when using such long wavelengths, since often only a single wavelength fringe circle is available with respect to a given point on the structural characteristics for imaging purposes. A single fringe circle from a point source is impossible to reconstruct optically because there exists no diffraction pattern or lens for holographic reconstruction purposes. Thus, an image of a point defect under these conditions is negated. This restricted aperture occurs in many applications where the defect is either near the surface or confined by geometry.
In the article titled "Holography by Scanning" by B. P. Hildebrand and Kenneth Haines, J. Opt. Soc. Am., Vol. 59, pages 1-19 (1969), there is a general discussion of the currently known techniques of imaging relating to scanned acoustic holography. The image location equations and magnifications discussed with respect to this disclosure were derived from a phase multiplication factor which appears to synthetically reduce the construction wavelength to simulate such previously known holographic techniques.
U.S. Pat. No. 4,084,136 to Libby et al discloses an eddy current testing device which produces a display of variations in characteristics of a sample. A signal expander samples a generated signal and expands the sample signal on a selected basis of square waves or Walsh functions to produce a plurality of signal components representative of the sampled signal. These are combined by a circuit network to provide a display of a defect. The initial signals are lissajou patterns which are then projected and rotated to provide a line image of a subsurface defect. While a visual display is achieved, an accurate representation of the size and dimensions of the detected flaw does not result.
In U.S. Pat. No. 3,721,896 to Mori et al, an eddy current testing technique is described in which the output signal from the object is processed to double the frequency and phase of the signal to produce a reference phase signal. The doubled reference phase is compared with a reference phase developed by a phase shifter to increase the sensitivity of a synchronous comparator. The system determines the difference between the two phases and results in a final output signal representing the detected difference. While this disclosure identifies the possibility of increasing the sensitivity of the eddy current testing technique, it contains no suggestion of utilizing such a signal to produce fringe patterns containing holographic information for further processing.
U.S. Pat. No. 4,005,358 to Foner discloses a magnetometer wherein distortions in the measured magnetic moment due to eddy currents are eliminated by the cancellation of out-of-phase signals through an alternating current feedback network. There is no discussion of potential phase multiplication or holographic imaging.
Another prior patent directed to the task of increasing information available from eddy current testing devices is U.S. Pat. No. 3,229,918 to Libby. The invention disclosed employed at test coil excited by a multifrequency signal. The resulting outputs of the apparatus are analog signals representative of object characteristics. No imaging techniques are disclosed. Prior U.S. Pat. 4,207,520 to Flora et al describes an analog to digital type of balance system dealing with eddy current test techniques. Output signals are processed in a computer to provide phase sensitive detection of flaws such as cracks, but no imaging is discussed.
U.S. Pat. No. 3,678,452 to Silverman relates to acoustic holography and utilizes a frequency multiplier for increasing the described sampling techniques. This subdivides the coherent wave period into a number of phase integrals, each representative of a corresponding step in a final hologram parameter such as gray-scale density. Phase multiplication techinques for synthetically producing an image reproducable by holographic techniques are not discussed.
In U.S. Pat. No. 4,222,273 to Takahashi et al, there is described a holographic apparatus for detecting and imaging flaws in objects. A hologram is displayed in a fringe pattern and the position and shape of the object can be determined from that pattern by reproduction techniques. Frequency dividing techniques are used, but synthetic multiplication of detected phase signals is not described.